Cold rolling method

ABSTRACT

The present invention relates to a method for cold-rolling metallic rolling stock ( 4 ), in which the rolling stock ( 4 ) passes through the roll nip ( 3 ) between oppositely driven rollers ( 2 ) at room temperature in order to undergo a plastic shape change. To avoid the problems caused by the use of liquid coolants and to achieve an improved surface quality of the rolling stock ( 4 ), the invention proposes that inert gas, which is at a lower temperature than the rolling-stock temperature in the roll nip, is blown into the region of the roll nip ( 3 ). The invention also relates to a cold-rolling stand for carrying out this method.

The invention relates to a method for cold-rolling metallic rollingstock, in which the rolling stock passes through the roll nip betweenoppositely driven rollers at room temperature in order to undergo aplastic shape change. The invention also relates to a rolling stand forcarrying out the rolling method according to the invention.

Cold-rolling is a process which has long been known for the shaping ofcontinuously moving strip, profiled section or sheet made from steel orother metals. The process involves cold forming, during which—unlike inthe case of hot forming—the rolling stock is not heated prior to theactual forming operation, i.e. is subjected to the plastic deformationat the prevailing ambient temperature (room temperature). This change inshape at below the respective recrystallization temperature of themetals results in advantageous changes to the properties of the deformedmaterials, for example an increase in the strength and hardness.Moreover, it is as a result possible to produce material surfaces withdefined roughness values R_(a), specifically both the highest surfacequalities, with a roughness average in accordance with DIN 4768/1R_(a)<0.3 μm—crack and pore free (RP) or crack and pore free andbrightly shining (RPG)—and roughened surfaces with R_(a)>1.5 μm.Consequently, the surfaces can be optimally adapted to the requirementsfor subsequent processing steps. In principle, all cold-formable metalmaterials can be processed in this way, i.e. steel, nonferrous metals,aluminum and other alloys. For example, cold-rolled steel sheet iseminently suitable for direct further processing even with the highestquality demands, for example those encountered in the automotiveengineering sector.

To produce such high, defined surface qualities, harmful influenceswhich can lead to an undefined roughening of the surface have to be asfar as possible ruled out or controlled by suitable measures. As therolling stock passes through the roll nip, harmful influences of thistype result, inter alia, from the fact that the material surface and theroll surfaces, in the region of their contact surface outside theneutral point, are in principle at different web velocities, which leadsto mechanical frictional loads on the surfaces. The frictional heatwhich is generated, together with the evolution of heat caused byinternal friction as a result of the deformation energy supplied, leadsto considerable heating of the rolling stock in the roll nip. Thisthermal loading of the material additionally promotes adverse effects onthe surface caused by changes in the materials properties and byoxidation.

In the prior art, the abovementioned mechanical and thermal loads on thestrip surface are combated by using coolants which are liquid at roomtemperature. Before it enters the roll nip, the rolling stock iscontinuously wetted with water, oil or emulsions. As a result, therolling stock is simultaneously cooled and lubricated, so that therequired surface qualities can be produced.

However, a significant drawback of the above-mentioned liquid coolantsis that, during rolling, to some extent they remain on the surface,where they have adverse effects. For example, water and water-containingemulsions lead to corrosion, i.e. to the formation of rust in the caseof steel sheet or strip. Oil and oil-containing emulsions leave residuesof oil on the surface, and these have to be removed again, as far aspossible without leaving any residues, prior to the further processingby welding, electrochemical surface treatment or the like, using furtheroperations, which are relatively complex and often environmentallypolluting. Of course, this entails very considerable outlay in terms oflabor, time and cost.

In view of the above, the present invention is based on the object ofproviding a cold-rolling method and a cold-rolling stand for carryingout this method which as far as possible avoids the above problemscaused by the use of conventional coolants. In particular, it isintended to ensure sufficient cooling and lubrication in the roll nip,while as far as possible there should be no harmful residues remainingon the rolling stock.

To solve the above problems, the method according to the inventionproposes that inert gas, which is at a lower temperature than therolling-stock temperature in the roll nip, is blown into the region ofthe roll nip.

According to the method according to the invention, the roll nip or therolling stock which is passing through the roll nip has inert gaslocally passing around it. The inert gas used is a nonoxidizing gas, forexample nitrogen, noble gases, carbon dioxide or other gases and gasmixtures which do not attack the surface of the rolling stock, i.e. donot cause any corrosion to this surface. According to the invention,this inert gas should be cooler than the rolling stock in the roll nip.This means that the inert gas temperature should be at least below themaximum temperature of the material which occurs during the deformationin the roll nip. Since this temperature of the material, on account ofthe thermal influences which have been outlined above, is higher thanambient temperature even during cold-rolling, the method according tothe invention is effective even if the inert gas temperature is at orslightly below ambient temperature (room temperature).

The method according to the invention is based on the surprisingdiscovery that a targeted stream of inert gas simultaneously produceseffective dissipation of heat from the roll nip, a corrosion-inhibitingaction and, a particularly unexpected phenomenon, considerable reductionin the friction in the roll nip. This means that, according to theinvention, gas cooling and lubrication is achieved for the first time inthe cold-rolling process.

The inert gas which is blown in in the region of the roll nip locallyforms a protective atmosphere in the region of the roll nip, whichreliably prevents corrosion, for example oxidation of the surfaces ofthe rolling stock and also of the roll surfaces in the region of theroll nip. Unlike conventional, liquid coolants, the inert gas providesparticularly good protection against oxidation, on account of the factthat the ambient air is displaced without leaving any residual air.

As a result of the temperature gradient with respect to the rollingstock, which is locally heated in the roll nip during deformation, theinert gas which is flowing past and is cooler than the rolling stock inthat region produces effective cooling of the rolling stock in thedirect region of the roll nip. Consequently, the thermal loads on thesurfaces fall in that region. This gas cooling is probably particularlyeffective because the cooling gas penetrates a relatively long way intothe roll nip between roll surface and rolling-stock surface.

Surprisingly, it has also emerged that the inert gas which is blown inaccording to the invention reduces the friction between the roll surfaceand the rolling-stock surface to such an extent that additionallubrication is no longer required. One possible explanation for thisunexpected, positive lubrication effect is based on the possibility thata microscopically thin film of the inert gas is adsorbed on therolling-stock surface, which is cooled by the inert gas flowing past it,and possibly also on the roll surface. As a result, it appears that atype of gas cushion is formed in the roll nip, i.e. at the point ofcontact between the rolling-stock surface and the roll surface, so thatan improved lubricating effect is produced compared to the use of liquidlubricants which has hitherto been customary.

The method according to the invention therefore demonstrates for thefirst time a way of replacing the coolants which are liquid at roomtemperature and have hitherto been considered imperative, such as water,oil or emulsions, with a cooling gas which is in gas form at roomtemperature.

The particular advantages of the method according to the inventionresult from the fact that all the drawbacks of liquid coolants arecompletely eliminated. In particular, the inert cooling gas does notleave behind any harmful residues whatsoever on the rolling stock, andconsequently there is no longer any need for any separate operations fordegreasing, removal of rust or the like prior to the further processing.Rather, the rolling stock can be directly processed further immediatelyafter rolling, for example by welding, electrochemical surfacetreatment, enameling or deformation or the like. Moreover, the inert gassuppresses oxidation phenomena much more effectively than would bepossible with known coolants.

The fact that the service life of the working rolls, in particular forthe highest surface quality RPG (free of cracks and pores, brightlyshining), is considerably increased has proven to be a further,extremely positive additional effect. This is, of course, particularlyadvantageous since the rolls have to be replaced and reworkedcorrespondingly less frequently. The same is true of surface qualitieswith higher, defined roughnesses, i.e. the predetermined R_(a) valuescan be reproduced for a longer time.

Preferably, the inert gas is blown in such a way that it is directedonto the boundary of the contact surface in the roll nip between rollingstock and roller. This controlled injection of the inert gas into theregions where the rolling stock enters and leaves the roll nip resultsin particularly good local cooling of the rolling stock at locationswhere the maximum thermal loads occur. Furthermore, it is ensured thatatmospheric oxygen which is entrained by the roll and rolling-stocksurfaces is reliably displaced and is not carried into the roll nip.Furthermore, the lubricating action of the gas lubrication according tothe invention is also improved by the directed blowing onto the edge ofthe boundary surface.

The inert gas is preferably blown in at the rolling-stock entry and atthe rolling-stock exit. This ensures particularly good cooling andreliable shielding from harmful atmospheric oxygen. In individual cases,however, it may even be sufficient for the inert gas to be supplied atthe rolling-stock entry or at the rolling-stock exit.

The inert gas is expediently supplied at least on the top side of therolling stock. This arrangement exploits the fact that the cold inertgas is heavier than ambient air and therefore also flows around theunderside of the rolling stock and the lower roll purely under theinfluence of the force of gravity.

As has been explained above, for cooling purposes the inert gas shouldat least be at room temperature; even at this temperature, it is coolerthan the rolling stock, which is at a higher temperature than roomtemperature in the roll nip. However, the advantageous effects withregard to cooling and lubrication are further improved by the inert gastemperature being below room temperature. Even slight cooling hasnoticeable positive effects which, of course, is particularlyadvantageous with regard to relatively great roll widths with arelatively high demand for cooling gas. However, the lower the inert gastemperature, the more the inventive advantages come to bear. Therefore,if quality requirements demand, cryogenic gas at a temperature ofapproximately −60° C. to −150° C. is used.

A particularly advantageous embodiment of the method according to theinvention provides for the inert gas to be blown at below itsliquefaction temperature. The inert gas, for example nitrogen, which isin gas form under standard conditions (room temperature, standardpressure) is in this case cooled to such an extent that it adopts theliquid state of aggregation. It is then blown or injected, in accordancewith the method according to the invention, into the region of the rollnip in the form of a liquefied gas. Unlike the known coolants which areliquid at room temperature, this liquefied gas, when it is heated toroom temperature, passes into the gaseous state of aggregation withoutany residues, and consequently leaves no more harmful residues on therolling stock than if it had been blown in in gas form.

The considerably improved cooling action when using liquefied gasresults from its extremely low temperature and from the fact that itextracts all its evaporation energy for transition into the gaseousstate of aggregation as thermal energy from the environment, with theresult that relatively large amounts of heat are dissipated from therolling stock within a short time. Consequently, the rolling stockenters the roll nip at a very low temperature. The heat of deformationwhich is generated in the roll nip is dissipated almost immediately atthe rolling-stock exit by the liquefied-gas cooling. The thermal load onthe surfaces, specifically both on the rolling-stock surfaces and theroll surfaces, is in this way reduced to a minimum. Moreover, thedifferences in temperature result in the formation of a gas cushion onthe contact surface in the roll nip, so that the rolling friction andtherefore the mechanical loads on the surfaces are likewise greatlyreduced. Finally, the low surface temperatures effectively reduce thesurface corrosion caused by oxidation, even if the rolling stock or theroll surface leaves the region around the roll nip to which inert gas issupplied directly.

Initial tests have shown that the use of the liquefied gas, specificallyliquid nitrogen, under otherwise identical conditions results in asudden improvement in the quality of the strip surface from RP (free ofcracks and pores) to RPG (free of cracks and pores, brightly shining).At the same time, the service life of the rolls is extended by amultiple. The effect whereby the roll surface becomes matt after acertain time, as has hitherto been observed but cannot be accepted forthe qualities RP and RPG and the causes of which are as yet unclear,likewise no longer occurs in the method according to the invention.

The method according to the invention is preferably carried out for thecold-rolling of steel, in particular strip steel and steel sheet, andspecifically in particular for high surface qualities in accordance withDIN EN 10139. However, the method according to the invention is notrestricted to the processing of steel, but rather may, of course, alsobe used for the cold-rolling of other cold-formable metal materials, forexample of nonferrous metals, aluminum and further metals and alloys.

To summarize, it can be stated that the invention for the first timeshows a possible way of completely replacing the coolants which areliquid at room temperature and have hitherto been customary with acooling gas which is in gas form at room temperature. The particularadvantages result from the fact that all the problems which havehitherto been caused by the coolants themselves are eliminated and, atthe same time, the surface quality achieved during the cold-rolling isconsiderably improved, practically without any additional outlay.

The method according to the invention can be implemented with relativelylittle design outlay on a cold-rolling stand for the cold-rolling ofmetallic rolling stock, which has at least two rolls (working rolls)which are mounted in a rolling frame in such a manner that they can bedriven in opposite directions and between which the roll nip, throughwhich the rolling stock passes, undergoing a change in shape, islocated. According to the invention, this stand has nozzles which can besupplied with cold inert gas and which are directed at the roll nip.

The inert gas can be blown into the region of the roll nip over theentire width of the rolling stock through these nozzles, with their gasoutlet preferably oriented substantially tangentially with respect tothe roll surface, i.e. the inert gas can, as described, be blown ontothe boundary of the contact surface between the rolling-stock surfaceand the roll surface.

The nozzles are expediently arranged at the rolling-stock entry and atthe rolling-stock exit. They should be arranged at least on the top sideof the rolling stock. This arrangement is often sufficient, since thecold inert gas will flush around the underside of the rolling stockpurely under the force of gravity. However, if appropriate it is alsopossible for nozzles to be arranged on the underside of the rollingstock.

Depending on the required cooling action and surface condition of therolling stock, it is possible to supply cryogenic gas or liquefied gasto the nozzles.

Another advantageous alternative or refinement of the present inventionprovides for a reactive gas, which can undergo chemical reactions withthe surface of the rolling stock in order to achieve defined surfaceproperties, to be used instead of the inert gas, which is passive withrespect to the surfaces of the material. The use of this reactive gaslikewise avoids the problems caused by the use of liquid coolants whichform the starting point for the invention. Specifically, the reactivegas can likewise produce sufficient cooling and lubrication in the rollnip, without any harmful residues remaining on the rolling stock, in thesame way as when using inert gas.

The effects relating to the gas cooling and lubrication according to theinvention of the rolling stock in the roll nip, which have beenexplained above and have thus far not been scientifically explained withany definitive reliability, can likewise be achieved by the reactivegas, which is supplied at a cool temperature. The atmospheric oxygen,which may have harmful effects on the surface properties of the rollingstock and of the rolls in the roll nip, is displaced by the reactivegas. In addition to these effects, which can already be achieved withinert gas, still further advantages can be achieved through the use ofreactive gas. For example, it is conceivable for the reactive gas, whichis initially supplied in cooled form, to react in a controlled way withthe material surface of the rolling stock as a result of the heating inthe roll nip. In this way it would, for example, be conceivable to applyreactively applied protective layers to the surface of the rollingstock, specifically in particular before the rolling stock comes intocontact with the ambient air. This could provide particular advantagesin particular with reactive metals, such as for example aluminum.

Suitable reactive gases are all gases or gas mixtures which, undersuitable conditions, for example in defined temperature ranges, canreact in a predeterminable way with the corresponding material of therolling stock. For example, it is conceivable to use carbon dioxide andother inorganic or organic gases or gas mixtures.

A cold-rolling stand according to the present invention is explained inmore detail below with reference to the drawings, in which, in detail:

FIG. 1 shows a diagrammatic perspective view of a rolling standaccording to the invention;

FIG. 2 shows a side view of the rolling stand shown in FIG. 1.

FIG. 1 diagrammatically depicts a perspective view, at an angle fromabove, of a cold-rolling stand according to the invention, in which therolling frames have been omitted for the sake of clarity. Thiscold-rolling stand, which is denoted overall by reference 1, has tworolls 2 which are arranged vertically above one another and betweenwhich the roll nip 3 is located.

In the illustration, the rolling stock is formed by a metal strip orsheet 4, for example of steel, which is passing through the roll nip 3in the direction indicated by the arrow.

Reference 5 denotes nozzles which are arranged on the strip entry sideand the strip exit side of the rolling stand 2 and the gas outlets ofwhich are directed obliquely from above into the region of the roll nip3.

The arrangement of the individual parts can be seen once again,particularly clearly, from the side view shown in FIG. 2. This figureindicates additional nozzles 5 on the underside of the rolling stock 4,which are likewise directed at the roll nip 3.

A cryogenic fluid may be injected or blown into the boarder of thecontact surface of the roll nip according to one embodiment the presentinvention. This cryogenic fluid may, as has been disclosed above, be ineither a gaseous or liquid phase when injected by the nozzle. Thecryogenic fluid, selected according to one embodiment of the presentinvention, is a gas at ambient or room temperature. This injectionprovides at least three benefits: it lubricates the roll nip and therolling stock; it cools the roll nip and the rolling stock at a contactsurface between the roll nip and the rolling stock; and it displacesambient gasses from the contact surface.

To operate the cold-rolling stand 1, the rolls 2 are driven in rotationin a known way, with the result that the rolling stock 4 passes throughthe roll nip 3, undergoing a change in shape as it does so. Inaccordance with the invention, inert gas, specifically, for preference,cold or cryogenic gas or liquefied gas, for example nitrogen, issupplied to the nozzles 5. This results in effective local cooling ofthe rolling stock 4 in the region of the roll nip 3. The localprotective atmosphere in the region of the roll nip 3 reliably preventsoxidation. The inert gas immediately ensures reliable lubricationbetween the roll and rolling-stock surfaces, so that there is no needfor any further coolants or lubricants.

It is also possible to use reactive gas instead of chemically passiveinert gas. This gas is distinguished by a defined chemical reactivitywith the material of the rolling stock. In addition to the gas coolingand lubrication according to the invention without the use of liquidcoolants, it is in this way possible to deliberately influence thesurface during the rolling operation, for example to form surfaceprotective layers.

1. A method for cold-rolling metallic stock, said method comprising:passing rolling stock through a roll nip disposed between two oppositelydriven rollers; injecting a cryogenic fluid, in a liquid phase, into acontact surface of said rolling stock immediately adjacent to said rollnip, said cryogenic fluid being a gas at ambient temperature;lubricating said roll nip and said rolling stock with said cryogenicfluid; cooling said roll nip and said rolling stock at a contact surfacebetween said roll nip and said rolling stock with said cryogenic fluid;and inducing a plastic shape change in said rolling stock.
 2. The methodaccording to claim 1 wherein said cryogenic fluid is injected across thefull width of the rolling stock at the roll nip.
 3. The method accordingto claim 1 wherein said cryogenic fluid is inert and further comprisingthe step of displacing ambient gasses from between said contact surfaceand said roller at said roll nip with said cryogenic fluid.
 4. Themethod as claimed in claim 1, characterized in that the fluid isinjected substantially tangentially with respect to a roll surface of atleast one said roller.
 5. The method as claimed in claim 1,characterized in that the fluid is injected at the rolling-stock entryand/or at the rolling-stock exit.
 6. The method as claimed in claim 1,characterized in that the fluid is supplied at least on the top side ofthe rolling stock.
 7. The method as claimed in claim 1, characterized inthat the fluid temperature is at least at or below room temperature. 8.The method as claimed in claim 1, characterized in that said fluid isnitrogen.
 9. The method as claimed in claim 1, characterized in thatsaid fluid is a noble gas.
 10. The method as claimed in claim 1,characterized in that said fluid comprises a mixture of fluids.
 11. Themethod as claimed in claim 1, characterized in that the rolling stock isstrip steel.
 12. The method as claimed in claim 1, characterized in thatthe rolling stock is steel sheet.
 13. The method as claimed in claim 1,further comprising imparting an average roughness (Ra) of 0.03 to 1.5 μm(in accordance with DIN 4768/1) to said rolling stock.
 14. A method forcold-rolling metallic stock, said method comprising: (a) admitting asheet of rolling stock into an inlet side of a roll nip disposed betweentwo oppositely driven rollers; (b) injecting a steam of cryogenic fluid,in a liquid phase, on at least said inlet side of said roll nip ontosubstantially the full width of at least one contact surface of saidsheet immediately adjacent said roll nip and substantially tangent tothe surface of the respective said roller; (c) lubricating said contactsurface of said sheet and said surface of said roller in said roll nipwith said cryogenic fluid; (d) cooling said contact surface of saidsheet and said surface of said roller in said roll nip with saidcryogenic fluid; and thereby (e) inducing a plastic shape change in saidsheet.
 15. The method according to claim 14, wherein said cryogenicfluid is substantially free of oxidizing agents and is converted to agaseous state by cooling said contact surface and further comprising thestep of displacing ambient gasses from between said contact surface andsaid roller at said roll nip with said cryogenic fluid.